Hello Ram,

JEE mains exam pattern has always followed the NCERT consistently over several years. So it is important that you stick to the syllabus and read the NCERT's of Mathematics, Chemistry and Physics thoroughly. Let us take one subject at a time.
CHEMISTRY:NCERT for chemistry is very important as there are several times when JEE has asked direct questions from it. For organic, read all the reactions thoroughly, memorise the reagents and catalysts and clear your concepts regarding their use. The formulae in must be on your fingertips. The physical and inorganic portion must also be thoroughly read. After this you must attempt questions from past 10-12 years of jee mains. Several books are available in market like Arihant, McGraw Hill,etc that provide solved previous year papers.
MATHEMATICS: Practice is the key to maths. Once you have covered the syllabus from NCERT, practice questions from books like R D Sharma or TMH Mathematics. Also refer SL loney and Hall& Knight. Honestly attempt the previous 10-12 year papers to enhance your understanding of concepts. Again, the formulae must be on your fingertips.
PHYSICS: Physics is a very conceptual subject. Don't try to mug up things without understanding, as this will be time consuming and not fruitful. Its better to read less but whatever you read is conceptually clear to you. Some of the important books apart from NCERT are H C Verma and Arihant DC Pandey. H C Verma is a must solve book for jee mains. Lastly solve the past years papers for the final touches to your preparation.
One of the most important thing in preparation is joining a full test series of any good coaching institute. Do not underestimate its importance as it will give you an experience in solving jee mains paper which will be really helpful when you sit for the real exam. It will also increase your problem solving speed which is on of the major rank deciding factors.
Don't panic, relax, work hard, be patient and you will easily clear mains.